1. Field of the Invention
The invention relates to a cage in which a plurality of pockets for retaining rollers on a one-to-one basis are provided at a plurality of locations in a circumferential direction, a production method for the cage, and a thrust roller bearing equipped with the cage.
2. Description of the Related Art
As shown in FIG. 4, a thrust roller bearing is composed of a pair of race members 60, 70, a plurality of rollers 10 interposed between the two race members 60, 70, and a cage 20 that retains the plurality of rollers 10 with predetermined intervals left therebetween in the circumferential direction. This cage 20 has, for example, a radially inward-side annular plate 40, a radially outward-side annular wall 51, and a plurality of pillar portions 30 between the annular plate 40 and the annular wall 51. The cage 20 is formed so as to have a generally letter “W” sectional shape. Pockets 31 are formed between the pillar portions 30 that are adjacent to the radially inward-side annular plate 40 and to the radially outward-side annular wall 51. Besides, stopper protrusions 33a, 33b are protruded into each pocket 31 from surfaces 33 of adjacent pillar portions 30 that face each other, specifically, from a radially intermediate portion and two radially end portions of each surface 33. This structure has been known in the field. The cage 20 constructed in this manner is produced by a press process of forming the cage 20 and, simultaneously with or after the press process, a blanking process in which the pockets 31 are formed by blanking. Besides, simultaneously with the blanking process, the stopper protrusions 33a, 33b are formed. This kind of thrust roller bearing is described in, for example, Japanese Patent Application Publication No. 10-252654 (JP-A-10-252654).
However, in the above-described cage 20, the occurrence of so-called chipping in which in the blanking process, a blanking punch 90 scrapes an outer peripheral surface 40a of the radially inward-side annular plate 40 or an inner peripheral surface 51a of the radially outward-side annular wall 51 is prevented. Therefore, as shown in FIG. 5, the outside diameter of the blanking punch 90 is slightly smaller than the distance between the outer peripheral surface 40a of the radially inward-side annular plate 40 and the inner peripheral surface 51a of the radially outward-side annular wall 51. The provision of such a difference for securing a clearance between the blanking punch 90 and both the outer peripheral surface 40a of the radially inward-side annular plate 40 and the inner peripheral surface 51a of the radially outward-side annular wall 51, thereby preventing the occurrence of the above-described chipping. Therefore, as shown in FIG. 4, inner wall surfaces 42, 52 formed by blanking the pockets 31 have portions that are protruded relative to the outer peripheral surface 40a of the radially inward-side annular plate 40 and the inner peripheral surface 51a of the radially outward-side annular wall 51, respectively. Due to the presence of the portions protruded relative to the outer peripheral surface 40a of the radially inward-side annular plate 40 or the inner peripheral surface 51a of the radially outward-side annular wall 51, when the thrust roller bearing is connected to, for example, a main shaft of a swash plate type compressor (not shown), and the swash plate type compressor is driven, the rollers 10 are given centrifugal force due to the revolution of the rollers 10, so that as shown in FIG. 6, the rollers 10 shift in the pockets 31 to the radially outward-side. As a result, of the two inner wall surfaces 42, 52, the inner wall surface 52, which is located at the radially outward-side, are partially in contact with radially outward-side end surfaces 10a of the rollers 10. This contact state causes a problem of abrasion occurring on the inner wall surface 52 and the radially outward-side end surfaces 10a of the rollers 10 (i.e., abrasion of a sold-shaded portion in an enlarged partial view in FIG. 6). In order to solve this problem, it has been considered that the pockets 31 are blanked so that as shown in FIG. 7 the inner peripheral surface 51a of the radially outward-side annular wall 51 is flush with the radially outward-side inner wall surface 52 formed by blanking the pockets 31. However, in order to perform the blanking in this manner, the pockets 31 needs to be blanked out closely along the inner peripheral surface 51a of the radially outward-side annular wall 51. Thus, there is difficulty from the viewpoint of preventing the foregoing chipping.